DN13

Design Note 13
Issue 2 June 1995
High Efficiency Hybrid
DC-DC Converter
+ 12V
0.1uF
Output
(2W)
0V
W1 28T
W2
W3
T1
10T
10T
+ 5V
0.1uF
560
Input
560
W4
ZTX
449
4T
ZTX
449
0V
To meet the growing requirement of
DC-DC converters that fit into 0.6 inch
pitch DIL IC packages (for LAN hub
power supplies for example), efficient
but low cost hybrid circuits must be
used. Using low switching loss bipolar
transistors from Zetex, converters such
as the cir cuit s hown here can be
constructed on small substrates giving
p o w e r d e n s i t i e s o f o v e r 1 . 7 W / cc .
Operating at over 80kHz the toroidal
transformer and ceramic decoupling
capacitor volumes are minimised, while
the consistency of the hFE process
provides a balanced switching
performance with no worry of low
tempera ture start-up or transistor
induced shoot-through.
The ZTX449 devices, possessing a
saturation voltage of only 150mV at 0.5A
give the 2W converter an efficiency of
over 77%, thus keeping power losses
and internal heating at a low level.
Maximum operating temperature is
ultimately limited by the magnetics but
the standard industrial range of -20°C to
+85°C is easily covered. Other devices
are available that exhibit higher gain
and/or higher voltage ratings for other
supply voltage options.
DN13-1
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